Isn't farming with E a form of laning safe?
No. E still has rather low range, so against champs like Xerath, Syndra, Yasuo... you simply cannot farm if they are near. Would be too easy for them to just poke you out of the lane.
Before, you sacrificed all lane presence and became essentially non-interacting, which is poor gameplay, when you maxed Q to passively farm.
Actually many of us used Q to harass, not only farm. In some match ups there was a really good reason why you wouldn't never max E first, like against certain assassins for example... you simply cannot be so close to them / trade with E, since they would just kill you.
Now, you can rank an ability that you can actually trade with and use it to help farm.
Nope, not really... until you get a mana item, your mana is simply not enough for you to do both. Either you farm and use your E to mana sustain, or you trade and go OOM.
The idea is whether or not she is conceptually in a healthier spot.
However she's not in a healthier spot. Current trash version of Cass is no way near healthier... not only does she encourage passive play from you and non interactive play from your opponent (ultimately denies Cass stacks), your passive makes you intentionally and purposefully prolong the game instead of ending it ASAP. There's no reason to pick champions that have zero impact during early/mid game besides ADC (and they have a support for a reason) and which also requires your team to carry team into the late game before they start doing something. One of the reasons why Poppy and Nasus are also a rare sight.